Editing Chrome plating (section)

==Mechanism of chromium electroplating==
It has been known for over a century, that chromium electroplating is relatively easy from (di)chromate solutions, but difficult from Cr<sup>3+</sup> solutions. Several theories have been proposed to explain this finding.

An earlier view suggested, that  an active Cr<sup>3+</sup> species (perhaps, with a [[ligand]] rather than water) forms initially from electroreduced Cr<sup>6+</sup>.<ref>{{cite journal | doi=10.1016/j.electacta.2018.07.114 | title=Determination of the chromium(III) reduction mechanism during chromium electroplating | date=2018 | last1=Del Pianta | first1=Dimitri | last2=Frayret | first2=Jérôme | last3=Gleyzes | first3=Christine | last4=Cugnet | first4=Cyril | last5=Dupin | first5=Jean Charles | last6=Le Hecho | first6=Isabelle | journal=Electrochimica Acta | volume=284 | pages=234–241 }}</ref><ref>{{cite journal | doi=10.1039/d0cs01165g | title=Electrochemical reduction of Cr(vi) in water: Lessons learned from fundamental studies and applications | date=2021 | last1=Stern | first1=Callie M. | last2=Jegede | first2=Temitope O. | last3=Hulse | first3=Vanessa A. | last4=Elgrishi | first4=Noémie | journal=Chemical Society Reviews | volume=50 | issue=3 | pages=1642–1667 | pmid=33325959 }}</ref>  This active Cr<sup>3+</sup> species can be reduced into metallic chromium relatively easy. However, the "active Cr<sup>3+</sup>" also undergoes within less than 1 second a transition into "inactive Cr<sup>3+</sup>", which is believed to be a polymeric hexa-aqua complex.<ref>{{cite journal |last1=Mandich |first1=N. V. |title=Chemistry & Theory of Chromium Deposition: Part I-Chemistry. |journal=Plating & Surface Finishing |date=1997 |volume=84 |issue=5 |pages=108–115 |url=https://www.nmfrc.org/pdf/p0597e.pdf |language=en |issn=0360-3164}}</ref> Some complexes of Cr<sup>3+</sup> with ligand other than water can undergo relatively fast electroreduction to metallic chromium, and they are used in chromate-free chromium plating methods.<ref>{{cite journal | doi=10.3390/coatings12030354 | doi-access=free | title=Mechanical Behaviour of Hard Chromium Deposited from a Trivalent Chromium Bath | date=2022 | last1=Guillon | first1=Robin | last2=Dalverny | first2=Olivier | last3=Fori | first3=Benoit | last4=Gazeau | first4=Celine | last5=Alexis | first5=Joel | journal=Coatings | volume=12 | issue=3 | page=354 }}</ref><ref>{{cite journal | doi=10.3390/coatings12101555 | doi-access=free | title=Advances on Cr and Ni Electrodeposition for Industrial Applications—A Review | date=2022 | last1=Okonkwo | first1=Bright O. | last2=Jeong | first2=Chaewon | last3=Jang | first3=Changheui | journal=Coatings | volume=12 | issue=10 | page=1555 }}</ref>

A different school of thought suggests, that the main problem with chromium plating from Cr<sup>3+</sup> solution is [[hydrogen evolution reaction]] (HER), and the role of chromate is to scavenge H<sup>+</sup> ions in a reaction that competes with H<sub>2</sub> evolution:

{{center|Cr<sub>2</sub>O<sub>7</sub><sup>2-</sup> + 14H<sup>+</sup> + 6e<sup>−</sup> → 2Cr<sup>3+</sup> + 7H<sub>2</sub>O}}

The shine of plated chrome depends on whether microscopic cracks in the plating are visible on the surface. The dull appearance of some chrome layers is due to continuous cracks that propagate through the whole plated metal layer, while bright deposits appear in the case of small microcracks that are confined to inner depth of the deposit. This HER side-reaction mechanism seems more acceptable by the electrochemistry community at present. Methods of plating  chromium from Cr<sup>3+</sup> solutions that rely on reversed current pulses have been commercialized (allegedly, to reoxidize the H<sub>2</sub>).<ref name="patent">{{cite journal | doi=10.1149/2.F04203if | title=Looking at Patent Law: Patenting a Trivalent Chromium Plating Invention: Obviousness Rejections – Not So Obvious | date=2020 | last1=Taylor | first1=E. Jennings | last2=Inman | first2=Maria | journal=The Electrochemical Society Interface | volume=29 | issue=3 | pages=33–38 | doi-access=free }}</ref><ref name="Safety">{{cite web |title=Hexavalent Chromium Safety |url=https://www.osha.gov/chromium-vi |website=Occupational Safety and Health Administration |accessdate=2024-06-13}}</ref><ref name="Industry">{{cite web |title=Industrial Hard Chrome Plating |url=https://www.hardchrome.com/ |website=Hard Chrome Plating |accessdate=2024-06-13}}</ref>